LONG-TERM CHANGES OF DAUGAVA RIVER ICE PHENOLOGY UNDER THE IMPACT OF THE CASCADE OF HYDRO POWER PLANTS

This paper presents the results of the study of long-term changes of Daugava River ice phenology, i.e. the freeze-up date, the break-up date, and the duration of ice cover from 1919/1920 to 2011/2012, under the impact of the cascade of hydro power plants. The long-term changes of ice phenology were determined by global climate warming at the turn of the 20^th and the 21^st centuries and anthropogenic activities after the year 1939. The Mann-Kendall test showed that the ice freeze-up date has a positive trend, while the ice break-up date and the duration of ice cover had negative trends. The changes were statistically significant. Data series covering twenty years before and after construction of the hydro power plants were used for assessing the impact of each hydro power plant on changes of Daugava River ice phenology parameters. The study results showed that the duration of ice cover was significantly longer in water reservoirs, i.e. the freezeup date was earlier and the break-up date was later. Downstream of dams duration of ice cover was shorter with later freeze-up dates and earlier break-up dates. The impact of hydro power plants on ice phenology parameters gradually decreased with distance down from the dams.